[PATCH 0/5] mm/rmap: Refactor try_to_unmap_one

[PATCH 0/5] mm/rmap: Refactor try_to_unmap_one

[Dev Jain]

In preparation for batching anonymous large folio unmapping to optimize it,
refactor try_to_unmap_one. This series refactors hugetlb, anon-lazyfree
and anon-swapbacked logic into their own functions, significantly
reducing the length of the huge try_to_unmap_one.

---
Split out from
https://lore.kernel.org/all/20260526063635.61721-1-dev.jain@arm.com/

mm-selftests pass on arm64, built on arm64 and x86.

Dev Jain (5):
  mm/rmap: convert page -> folio for hwpoison checks
  mm/rmap: Add try_to_unmap_hugetlb_one
  mm/rmap: refactor some code around lazyfree folio unmapping
  mm/rmap: refactor anon swapbacked folio unmap in try_to_unmap_one
  mm/rmap: add anon folio unmap dispatcher function

 include/linux/hugetlb.h |   1 +
 mm/rmap.c               | 425 +++++++++++++++++++++++-----------------
 2 files changed, 244 insertions(+), 182 deletions(-)

-- 
2.43.0

[PATCH 1/5] mm/rmap: convert page -> folio for hwpoison checks

[Dev Jain]

try_to_unmap() receives hugetlb folios only from the hwpoison path.
hugetlb_update_hwpoison() sets the hugetlb folio's head-page
hwpoison bit, and page_vma_mapped_walk() reports the hugetlb mapping at
the head PFN, so the previous PageHWPoison(subpage) check happened to
work for hugetlb.

For non-hugetlb folios, unmap_poisoned_folio() currently rejects large
folios before calling try_to_unmap(). Hence it is always the case that
if try_to_unmap_one() handles an hwpoisoned folio, then the head page is
marked with the poison bit.

Therefore, convert the poisoned subpage checks to folio_test_hwpoison().

No functional change intended, except that, while at it,
convert VM_BUG_* to VM_WARN_*.

Signed-off-by: Dev Jain <dev.jain@arm.com>
---
 mm/rmap.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index 9939400e77c79..dbb077f8443e3 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -2122,10 +2122,11 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
 			bool anon = folio_test_anon(folio);
 
 			/*
-			 * The try_to_unmap() is only passed a hugetlb page
-			 * in the case where the hugetlb page is poisoned.
+			 * The try_to_unmap() is only passed a hugetlb folio
+			 * in the case where the hugetlb folio contains a
+			 * poisoned page.
 			 */
-			VM_BUG_ON_PAGE(!PageHWPoison(subpage), subpage);
+			VM_WARN_ON_FOLIO(!folio_test_hwpoison(folio), folio);
 			/*
 			 * huge_pmd_unshare may unmap an entire PMD page.
 			 * There is no way of knowing exactly which PMDs may
@@ -2204,7 +2205,11 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
 		/* Update high watermark before we lower rss */
 		update_hiwater_rss(mm);
 
-		if (PageHWPoison(subpage) && (flags & TTU_HWPOISON)) {
+		/*
+		 * With TTU_HWPOISON, we only expect small folios or hugetlb
+		 * folios here for now.
+		 */
+		if (folio_test_hwpoison(folio) && (flags & TTU_HWPOISON)) {
 			pteval = swp_entry_to_pte(make_hwpoison_entry(subpage));
 			if (folio_test_hugetlb(folio)) {
 				hugetlb_count_sub(folio_nr_pages(folio), mm);
-- 
2.43.0

[PATCH 2/5] mm/rmap: Add try_to_unmap_hugetlb_one

[Dev Jain]

Simplify try_to_unmap_one() by separating the hugetlb parts into
try_to_unmap_hugetlb_one().

To understand the correctness of the refactoring, the following points
are noted:

1. try_to_unmap() is called for hugetlb folios only when they are
   hwpoisoned.

2. A hugetlb VMA cannot be mlocked.

3. page_vma_mapped_walk() returns at most one hugetlb mapping in a VMA,
   and that mapping points at the head PFN.

4. We won't ever process a softleaf entry that encodes a hugetlb folio;
   hugetlb folios are never swapped out, migration entries will be
   skipped (PVMW_MIGRATION not passed), and device-exclusive does not
   work for hugetlb.

5. The hwpoison entry is constructed from the poisoned folio, just as in
   the pre-refactor code. Any previous uffd-wp state is deliberately not
   preserved for the hwpoison entry.

6. TTU_HWPOISON is always present; for it to not be present, either the
   folio has to be in swapcache, or mapping_can_writeback() is true (see
   unmap_poisoned_folio), none of which is true for hugetlb folios.

7. Hugetlb uses separate counters from normal rss counters, therefore
   update_highwater_rss() need not be called.

While at it:

 - Change VM_BUG_* to VM_WARN_*.

 - Do not declare variables which are only used once.

 - Use huge_pte_dirty() instead of pte_dirty().

 - Add 3 VM_WARN_ON_ONCE asserting that TTU_HWPOISON should be present,
   pte_present() must be true, and the pfn derived from the huge pte
   must be that of the head of the hugetlb folio.

Except the BUG->WARN change, no functional change intended.

Suggested-by: David Hildenbrand <david@kernel.org>
Signed-off-by: Dev Jain <dev.jain@arm.com>
---
 include/linux/hugetlb.h |   1 +
 mm/rmap.c               | 177 +++++++++++++++++++++-------------------
 2 files changed, 96 insertions(+), 82 deletions(-)

diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index 4115076e4922a..bf7e163e3779d 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -1271,6 +1271,7 @@ static inline void hugetlb_count_sub(long l, struct mm_struct *mm)
 }
 
 pte_t huge_ptep_get(struct mm_struct *mm, unsigned long addr, pte_t *ptep);
+unsigned long huge_pte_dirty(pte_t pte);
 
 static inline pte_t huge_ptep_clear_flush(struct vm_area_struct *vma,
 					  unsigned long addr, pte_t *ptep)
diff --git a/mm/rmap.c b/mm/rmap.c
index dbb077f8443e3..934773dfa2f2a 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1978,6 +1978,95 @@ static inline unsigned int folio_unmap_pte_batch(struct folio *folio,
 				     FPB_RESPECT_WRITE | FPB_RESPECT_SOFT_DIRTY);
 }
 
+static bool try_to_unmap_hugetlb_one(struct folio *folio,
+		struct vm_area_struct *vma, unsigned long address, void *arg)
+{
+	DEFINE_FOLIO_VMA_WALK(pvmw, folio, vma, address, 0);
+	struct mmu_notifier_range range;
+	enum ttu_flags flags = (enum ttu_flags)(long)arg;
+	struct mm_struct *mm = vma->vm_mm;
+	const unsigned long hsz = huge_page_size(hstate_vma(vma));
+	bool ret = true;
+	pte_t pteval;
+
+	/*
+	 * The try_to_unmap() is only passed a hugetlb folio in the case
+	 * where the hugetlb folio is poisoned.
+	 */
+	VM_WARN_ON_FOLIO(!folio_test_hwpoison(folio), folio);
+	VM_WARN_ON_ONCE(!(flags & TTU_HWPOISON));
+
+	range.end = vma_address_end(&pvmw);
+	mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma->vm_mm,
+				address, range.end);
+	adjust_range_if_pmd_sharing_possible(vma, &range.start, &range.end);
+	mmu_notifier_invalidate_range_start(&range);
+
+	/* There is only a single mapping in a VMA. */
+	if (!page_vma_mapped_walk(&pvmw))
+		goto range_end;
+
+	address = pvmw.address;
+	pteval = huge_ptep_get(mm, address, pvmw.pte);
+	VM_WARN_ON_ONCE(!pte_present(pteval));
+	VM_WARN_ON_ONCE(pte_pfn(pteval) != folio_pfn(folio));
+
+	/*
+	 * huge_pmd_unshare may unmap an entire PMD page. There is no way of
+	 * knowing exactly which PMDs may be cached for this mm, so we must
+	 * flush them all. start/end were already adjusted above to cover this
+	 * range.
+	 */
+	flush_cache_range(vma, range.start, range.end);
+
+	/*
+	 * To call huge_pmd_unshare, i_mmap_rwsem must be held in write mode.
+	 * Caller needs to explicitly do this outside rmap routines.
+	 *
+	 * We also must hold hugetlb vma_lock in write mode. Lock order dictates
+	 * acquiring vma_lock BEFORE i_mmap_rwsem. We can only try lock here and
+	 * fail if unsuccessful.
+	 */
+	if (!folio_test_anon(folio)) {
+		struct mmu_gather tlb;
+
+		VM_WARN_ON(!(flags & TTU_RMAP_LOCKED));
+		if (!hugetlb_vma_trylock_write(vma)) {
+			ret = false;
+			goto walk_done;
+		}
+
+		tlb_gather_mmu_vma(&tlb, vma);
+		if (huge_pmd_unshare(&tlb, vma, address, pvmw.pte)) {
+			hugetlb_vma_unlock_write(vma);
+			huge_pmd_unshare_flush(&tlb, vma);
+			tlb_finish_mmu(&tlb);
+			/*
+			 * The PMD table was unmapped, consequently unmapping
+			 * the folio.
+			 */
+			goto walk_done;
+		}
+		hugetlb_vma_unlock_write(vma);
+		tlb_finish_mmu(&tlb);
+	}
+	pteval = huge_ptep_clear_flush(vma, address, pvmw.pte);
+	if (huge_pte_dirty(pteval))
+		folio_mark_dirty(folio);
+
+	pteval = swp_entry_to_pte(make_hwpoison_entry(folio_page(folio, 0)));
+	hugetlb_count_sub(folio_nr_pages(folio), mm);
+	set_huge_pte_at(mm, address, pvmw.pte, pteval, hsz);
+	hugetlb_remove_rmap(folio);
+	folio_put_refs(folio, 1);
+
+walk_done:
+	page_vma_mapped_walk_done(&pvmw);
+range_end:
+	mmu_notifier_invalidate_range_end(&range);
+	return ret;
+}
+
 /*
  * @arg: enum ttu_flags will be passed to this argument
  */
@@ -1993,7 +2082,6 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
 	enum ttu_flags flags = (enum ttu_flags)(long)arg;
 	unsigned long nr_pages = 1, end_addr;
 	unsigned long pfn;
-	unsigned long hsz = 0;
 	int ptes = 0;
 
 	/*
@@ -2007,8 +2095,6 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
 
 	/*
 	 * For THP, we have to assume the worse case ie pmd for invalidation.
-	 * For hugetlb, it could be much worse if we need to do pud
-	 * invalidation in the case of pmd sharing.
 	 *
 	 * Note that the folio can not be freed in this function as call of
 	 * try_to_unmap() must hold a reference on the folio.
@@ -2016,17 +2102,6 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
 	range.end = vma_address_end(&pvmw);
 	mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma->vm_mm,
 				address, range.end);
-	if (folio_test_hugetlb(folio)) {
-		/*
-		 * If sharing is possible, start and end will be adjusted
-		 * accordingly.
-		 */
-		adjust_range_if_pmd_sharing_possible(vma, &range.start,
-						     &range.end);
-
-		/* We need the huge page size for set_huge_pte_at() */
-		hsz = huge_page_size(hstate_vma(vma));
-	}
 	mmu_notifier_invalidate_range_start(&range);
 
 	while (page_vma_mapped_walk(&pvmw)) {
@@ -2111,66 +2186,13 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
 			const softleaf_t entry = softleaf_from_pte(pteval);
 
 			pfn = softleaf_to_pfn(entry);
-			VM_WARN_ON_FOLIO(folio_test_hugetlb(folio), folio);
 		}
 
 		subpage = folio_page(folio, pfn - folio_pfn(folio));
 		anon_exclusive = folio_test_anon(folio) &&
 				 PageAnonExclusive(subpage);
 
-		if (folio_test_hugetlb(folio)) {
-			bool anon = folio_test_anon(folio);
-
-			/*
-			 * The try_to_unmap() is only passed a hugetlb folio
-			 * in the case where the hugetlb folio contains a
-			 * poisoned page.
-			 */
-			VM_WARN_ON_FOLIO(!folio_test_hwpoison(folio), folio);
-			/*
-			 * huge_pmd_unshare may unmap an entire PMD page.
-			 * There is no way of knowing exactly which PMDs may
-			 * be cached for this mm, so we must flush them all.
-			 * start/end were already adjusted above to cover this
-			 * range.
-			 */
-			flush_cache_range(vma, range.start, range.end);
-
-			/*
-			 * To call huge_pmd_unshare, i_mmap_rwsem must be
-			 * held in write mode.  Caller needs to explicitly
-			 * do this outside rmap routines.
-			 *
-			 * We also must hold hugetlb vma_lock in write mode.
-			 * Lock order dictates acquiring vma_lock BEFORE
-			 * i_mmap_rwsem.  We can only try lock here and fail
-			 * if unsuccessful.
-			 */
-			if (!anon) {
-				struct mmu_gather tlb;
-
-				VM_BUG_ON(!(flags & TTU_RMAP_LOCKED));
-				if (!hugetlb_vma_trylock_write(vma))
-					goto walk_abort;
-
-				tlb_gather_mmu_vma(&tlb, vma);
-				if (huge_pmd_unshare(&tlb, vma, address, pvmw.pte)) {
-					hugetlb_vma_unlock_write(vma);
-					huge_pmd_unshare_flush(&tlb, vma);
-					tlb_finish_mmu(&tlb);
-					/*
-					 * The PMD table was unmapped,
-					 * consequently unmapping the folio.
-					 */
-					goto walk_done;
-				}
-				hugetlb_vma_unlock_write(vma);
-				tlb_finish_mmu(&tlb);
-			}
-			pteval = huge_ptep_clear_flush(vma, address, pvmw.pte);
-			if (pte_dirty(pteval))
-				folio_mark_dirty(folio);
-		} else if (likely(pte_present(pteval))) {
+		if (likely(pte_present(pteval))) {
 			nr_pages = folio_unmap_pte_batch(folio, &pvmw, flags, pteval);
 			end_addr = address + nr_pages * PAGE_SIZE;
 			flush_cache_range(vma, address, end_addr);
@@ -2211,14 +2233,8 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
 		 */
 		if (folio_test_hwpoison(folio) && (flags & TTU_HWPOISON)) {
 			pteval = swp_entry_to_pte(make_hwpoison_entry(subpage));
-			if (folio_test_hugetlb(folio)) {
-				hugetlb_count_sub(folio_nr_pages(folio), mm);
-				set_huge_pte_at(mm, address, pvmw.pte, pteval,
-						hsz);
-			} else {
-				dec_mm_counter(mm, mm_counter(folio));
-				set_pte_at(mm, address, pvmw.pte, pteval);
-			}
+			dec_mm_counter(mm, mm_counter(folio));
+			set_pte_at(mm, address, pvmw.pte, pteval);
 		} else if (likely(pte_present(pteval)) && pte_unused(pteval) &&
 			   !userfaultfd_armed(vma)) {
 			/*
@@ -2346,11 +2362,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
 			add_mm_counter(mm, mm_counter_file(folio), -nr_pages);
 		}
 discard:
-		if (unlikely(folio_test_hugetlb(folio))) {
-			hugetlb_remove_rmap(folio);
-		} else {
-			folio_remove_rmap_ptes(folio, subpage, nr_pages, vma);
-		}
+		folio_remove_rmap_ptes(folio, subpage, nr_pages, vma);
 		if (vma->vm_flags & VM_LOCKED)
 			mlock_drain_local();
 		folio_put_refs(folio, nr_pages);
@@ -2398,7 +2410,8 @@ static int folio_not_mapped(struct folio *folio)
 void try_to_unmap(struct folio *folio, enum ttu_flags flags)
 {
 	struct rmap_walk_control rwc = {
-		.rmap_one = try_to_unmap_one,
+		.rmap_one = folio_test_hugetlb(folio) ?
+				try_to_unmap_hugetlb_one : try_to_unmap_one,
 		.arg = (void *)flags,
 		.done = folio_not_mapped,
 		.anon_lock = folio_lock_anon_vma_read,
-- 
2.43.0

[PATCH 3/5] mm/rmap: refactor some code around lazyfree folio unmapping

[Dev Jain]

For lazyfree folio unmapping, after clearing the ptes we must abort the
operation if the folio got dirtied or it has unexpected references.

Refactor this logic into a function which will return whether we need
to abort or not.

If we abort, we restore the ptes and bail out of try_to_unmap_one.
Otherwise adjust the rss stats of the mm and jump to a label.

Also rename that label from "discard" to "finish_unmap"; the former
is appropriate in the lazyfree context, but the code following the label
is executed for other successful unmap code paths too, so 'discard' does
not sound correct for them.

No functional change intended.

Signed-off-by: Dev Jain <dev.jain@arm.com>
---
 mm/rmap.c | 87 +++++++++++++++++++++++++++++++------------------------
 1 file changed, 49 insertions(+), 38 deletions(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index 934773dfa2f2a..00b571c2a1bab 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -2067,6 +2067,52 @@ static bool try_to_unmap_hugetlb_one(struct folio *folio,
 	return ret;
 }
 
+static inline bool ttu_anon_lazyfree_folio(struct vm_area_struct *vma,
+		struct folio *folio)
+{
+	int ref_count, map_count;
+
+	/*
+	 * Synchronize with gup_pte_range():
+	 * - clear PTE; barrier; read refcount
+	 * - inc refcount; barrier; read PTE
+	 */
+	smp_mb();
+
+	ref_count = folio_ref_count(folio);
+	map_count = folio_mapcount(folio);
+
+	/*
+	 * Order reads for page refcount and dirty flag
+	 * (see comments in __remove_mapping()).
+	 */
+	smp_rmb();
+
+	if (folio_test_dirty(folio) && !(vma->vm_flags & VM_DROPPABLE)) {
+		/*
+		 * redirtied either using the page table or a previously
+		 * obtained GUP reference.
+		 */
+		folio_set_swapbacked(folio);
+		return false;
+	}
+
+	if (ref_count != 1 + map_count) {
+		/*
+		 * Additional reference. Could be a GUP reference or any
+		 * speculative reference. GUP users must mark the folio
+		 * dirty if there was a modification. This folio cannot be
+		 * reclaimed right now either way, so act just like nothing
+		 * happened.
+		 * We'll come back here later and detect if the folio was
+		 * dirtied when the additional reference is gone.
+		 */
+		return false;
+	}
+
+	return true;
+}
+
 /*
  * @arg: enum ttu_flags will be passed to this argument
  */
@@ -2263,47 +2309,12 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
 
 			/* MADV_FREE page check */
 			if (!folio_test_swapbacked(folio)) {
-				int ref_count, map_count;
-
-				/*
-				 * Synchronize with gup_pte_range():
-				 * - clear PTE; barrier; read refcount
-				 * - inc refcount; barrier; read PTE
-				 */
-				smp_mb();
-
-				ref_count = folio_ref_count(folio);
-				map_count = folio_mapcount(folio);
-
-				/*
-				 * Order reads for page refcount and dirty flag
-				 * (see comments in __remove_mapping()).
-				 */
-				smp_rmb();
-
-				if (folio_test_dirty(folio) && !(vma->vm_flags & VM_DROPPABLE)) {
-					/*
-					 * redirtied either using the page table or a previously
-					 * obtained GUP reference.
-					 */
-					set_ptes(mm, address, pvmw.pte, pteval, nr_pages);
-					folio_set_swapbacked(folio);
-					goto walk_abort;
-				} else if (ref_count != 1 + map_count) {
-					/*
-					 * Additional reference. Could be a GUP reference or any
-					 * speculative reference. GUP users must mark the folio
-					 * dirty if there was a modification. This folio cannot be
-					 * reclaimed right now either way, so act just like nothing
-					 * happened.
-					 * We'll come back here later and detect if the folio was
-					 * dirtied when the additional reference is gone.
-					 */
+				if (!ttu_anon_lazyfree_folio(vma, folio)) {
 					set_ptes(mm, address, pvmw.pte, pteval, nr_pages);
 					goto walk_abort;
 				}
 				add_mm_counter(mm, MM_ANONPAGES, -nr_pages);
-				goto discard;
+				goto finish_unmap;
 			}
 
 			if (folio_dup_swap(folio, subpage) < 0) {
@@ -2361,7 +2372,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
 			 */
 			add_mm_counter(mm, mm_counter_file(folio), -nr_pages);
 		}
-discard:
+finish_unmap:
 		folio_remove_rmap_ptes(folio, subpage, nr_pages, vma);
 		if (vma->vm_flags & VM_LOCKED)
 			mlock_drain_local();
-- 
2.43.0

[PATCH 4/5] mm/rmap: refactor anon swapbacked folio unmap in try_to_unmap_one

[Dev Jain]

Refactor anonymous swapbacked folio unmap to ttu_anon_swapbacked_folio().

No functional change intended.

Signed-off-by: Dev Jain <dev.jain@arm.com>
---
 mm/rmap.c | 105 ++++++++++++++++++++++++++++++++----------------------
 1 file changed, 62 insertions(+), 43 deletions(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index 00b571c2a1bab..ade78df5be2bd 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -2113,6 +2113,64 @@ static inline bool ttu_anon_lazyfree_folio(struct vm_area_struct *vma,
 	return true;
 }
 
+static inline void set_swp_pte_at(struct mm_struct *mm, unsigned long address,
+		pte_t *ptep, swp_entry_t entry, pte_t pteval, bool anon_exclusive)
+{
+	pte_t swp_pte = swp_entry_to_pte(entry);
+
+	if (anon_exclusive)
+		swp_pte = pte_swp_mkexclusive(swp_pte);
+
+	if (likely(pte_present(pteval))) {
+		if (pte_soft_dirty(pteval))
+			swp_pte = pte_swp_mksoft_dirty(swp_pte);
+		if (pte_uffd_wp(pteval))
+			swp_pte = pte_swp_mkuffd_wp(swp_pte);
+	} else {
+		/* Device-exclusive entry */
+		if (pte_swp_soft_dirty(pteval))
+			swp_pte = pte_swp_mksoft_dirty(swp_pte);
+		if (pte_swp_uffd_wp(pteval))
+			swp_pte = pte_swp_mkuffd_wp(swp_pte);
+	}
+
+	set_pte_at(mm, address, ptep, swp_pte);
+}
+
+static inline bool ttu_anon_swapbacked_folio(struct vm_area_struct *vma, struct folio *folio,
+		struct page *subpage, unsigned long address, pte_t *ptep,
+		pte_t pteval)
+{
+	bool anon_exclusive = folio_test_anon(folio) && PageAnonExclusive(subpage);
+	swp_entry_t entry = page_swap_entry(subpage);
+	struct mm_struct *mm = vma->vm_mm;
+
+	if (folio_dup_swap(folio, subpage) < 0)
+		return false;
+
+	/*
+	 * arch_unmap_one() is expected to be a NOP on
+	 * architectures where we could have PFN swap PTEs,
+	 * so we'll not check/care.
+	 */
+	if (arch_unmap_one(mm, vma, address, pteval) < 0) {
+		folio_put_swap(folio, subpage);
+		return false;
+	}
+
+	/* See folio_try_share_anon_rmap(): clear PTE first. */
+	if (anon_exclusive && folio_try_share_anon_rmap_pte(folio, subpage)) {
+		folio_put_swap(folio, subpage);
+		return false;
+	}
+
+	mm_prepare_for_swap_entries(mm);
+	dec_mm_counter(mm, MM_ANONPAGES);
+	inc_mm_counter(mm, MM_SWAPENTS);
+	set_swp_pte_at(mm, address, ptep, entry, pteval, anon_exclusive);
+	return true;
+}
+
 /*
  * @arg: enum ttu_flags will be passed to this argument
  */
@@ -2121,7 +2179,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
 {
 	struct mm_struct *mm = vma->vm_mm;
 	DEFINE_FOLIO_VMA_WALK(pvmw, folio, vma, address, 0);
-	bool anon_exclusive, ret = true;
+	bool ret = true;
 	pte_t pteval;
 	struct page *subpage;
 	struct mmu_notifier_range range;
@@ -2235,8 +2293,6 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
 		}
 
 		subpage = folio_page(folio, pfn - folio_pfn(folio));
-		anon_exclusive = folio_test_anon(folio) &&
-				 PageAnonExclusive(subpage);
 
 		if (likely(pte_present(pteval))) {
 			nr_pages = folio_unmap_pte_batch(folio, &pvmw, flags, pteval);
@@ -2295,8 +2351,6 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
 			 */
 			dec_mm_counter(mm, mm_counter(folio));
 		} else if (folio_test_anon(folio)) {
-			swp_entry_t entry = page_swap_entry(subpage);
-			pte_t swp_pte;
 			/*
 			 * Store the swap location in the pte.
 			 * See handle_pte_fault() ...
@@ -2317,47 +2371,12 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
 				goto finish_unmap;
 			}
 
-			if (folio_dup_swap(folio, subpage) < 0) {
-				set_pte_at(mm, address, pvmw.pte, pteval);
-				goto walk_abort;
-			}
-
-			/*
-			 * arch_unmap_one() is expected to be a NOP on
-			 * architectures where we could have PFN swap PTEs,
-			 * so we'll not check/care.
-			 */
-			if (arch_unmap_one(mm, vma, address, pteval) < 0) {
-				folio_put_swap(folio, subpage);
-				set_pte_at(mm, address, pvmw.pte, pteval);
-				goto walk_abort;
-			}
-
-			/* See folio_try_share_anon_rmap(): clear PTE first. */
-			if (anon_exclusive &&
-			    folio_try_share_anon_rmap_pte(folio, subpage)) {
-				folio_put_swap(folio, subpage);
+			if (!ttu_anon_swapbacked_folio(vma, folio, subpage, address,
+						pvmw.pte, pteval)) {
 				set_pte_at(mm, address, pvmw.pte, pteval);
 				goto walk_abort;
 			}
-			mm_prepare_for_swap_entries(mm);
-			dec_mm_counter(mm, MM_ANONPAGES);
-			inc_mm_counter(mm, MM_SWAPENTS);
-			swp_pte = swp_entry_to_pte(entry);
-			if (anon_exclusive)
-				swp_pte = pte_swp_mkexclusive(swp_pte);
-			if (likely(pte_present(pteval))) {
-				if (pte_soft_dirty(pteval))
-					swp_pte = pte_swp_mksoft_dirty(swp_pte);
-				if (pte_uffd_wp(pteval))
-					swp_pte = pte_swp_mkuffd_wp(swp_pte);
-			} else {
-				if (pte_swp_soft_dirty(pteval))
-					swp_pte = pte_swp_mksoft_dirty(swp_pte);
-				if (pte_swp_uffd_wp(pteval))
-					swp_pte = pte_swp_mkuffd_wp(swp_pte);
-			}
-			set_pte_at(mm, address, pvmw.pte, swp_pte);
+			goto finish_unmap;
 		} else {
 			/*
 			 * This is a locked file-backed folio,
-- 
2.43.0

[PATCH 5/5] mm/rmap: add anon folio unmap dispatcher function

[Dev Jain]

Add ttu_anon_folio() as the common entry point for anonymous folio
unmapping. It dispatches to the lazyfree or swapbacked helper as
appropriate, and centralizes restoration of cleared PTEs on failure.

No functional change intended.

Signed-off-by: Dev Jain <dev.jain@arm.com>
---
 mm/rmap.c | 59 +++++++++++++++++++++++++++++++++----------------------
 1 file changed, 36 insertions(+), 23 deletions(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index ade78df5be2bd..f021ecd51a4a2 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -2171,6 +2171,40 @@ static inline bool ttu_anon_swapbacked_folio(struct vm_area_struct *vma, struct
 	return true;
 }
 
+static inline bool ttu_anon_folio(struct vm_area_struct *vma, struct folio *folio,
+		struct page *subpage, unsigned long address, pte_t *ptep,
+		pte_t pteval, unsigned long nr_pages)
+{
+	struct mm_struct *mm = vma->vm_mm;
+	bool ret;
+
+	/*
+	 * Store the swap location in the pte.
+	 * See handle_pte_fault() ...
+	 */
+	if (unlikely(folio_test_swapbacked(folio) !=
+			folio_test_swapcache(folio))) {
+		WARN_ON_ONCE(1);
+		return false;
+	}
+
+	/* MADV_FREE page check */
+	if (!folio_test_swapbacked(folio)) {
+		ret = ttu_anon_lazyfree_folio(vma, folio);
+		if (ret)
+			add_mm_counter(mm, MM_ANONPAGES, -nr_pages);
+	} else {
+		/* nr_pages > 1 not supported yet */
+		ret = ttu_anon_swapbacked_folio(vma, folio, subpage, address,
+						ptep, pteval);
+	}
+
+	if (!ret)
+		set_ptes(mm, address, ptep, pteval, nr_pages);
+
+	return ret;
+}
+
 /*
  * @arg: enum ttu_flags will be passed to this argument
  */
@@ -2351,31 +2385,10 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
 			 */
 			dec_mm_counter(mm, mm_counter(folio));
 		} else if (folio_test_anon(folio)) {
-			/*
-			 * Store the swap location in the pte.
-			 * See handle_pte_fault() ...
-			 */
-			if (unlikely(folio_test_swapbacked(folio) !=
-					folio_test_swapcache(folio))) {
-				WARN_ON_ONCE(1);
+			if (!ttu_anon_folio(vma, folio, subpage, address,
+					    pvmw.pte, pteval, nr_pages))
 				goto walk_abort;
-			}
 
-			/* MADV_FREE page check */
-			if (!folio_test_swapbacked(folio)) {
-				if (!ttu_anon_lazyfree_folio(vma, folio)) {
-					set_ptes(mm, address, pvmw.pte, pteval, nr_pages);
-					goto walk_abort;
-				}
-				add_mm_counter(mm, MM_ANONPAGES, -nr_pages);
-				goto finish_unmap;
-			}
-
-			if (!ttu_anon_swapbacked_folio(vma, folio, subpage, address,
-						pvmw.pte, pteval)) {
-				set_pte_at(mm, address, pvmw.pte, pteval);
-				goto walk_abort;
-			}
 			goto finish_unmap;
 		} else {
 			/*
-- 
2.43.0

Re: [PATCH 1/5] mm/rmap: convert page -> folio for hwpoison checks

[David Hildenbrand (Arm)]

On 7/7/26 14:11, Dev Jain wrote:
> try_to_unmap() receives hugetlb folios only from the hwpoison path.
> hugetlb_update_hwpoison() sets the hugetlb folio's head-page
> hwpoison bit, and page_vma_mapped_walk() reports the hugetlb mapping at
> the head PFN, so the previous PageHWPoison(subpage) check happened to
> work for hugetlb.
> 
> For non-hugetlb folios, unmap_poisoned_folio() currently rejects large
> folios before calling try_to_unmap(). Hence it is always the case that
> if try_to_unmap_one() handles an hwpoisoned folio, then the head page is
> marked with the poison bit.
> 
> Therefore, convert the poisoned subpage checks to folio_test_hwpoison().
> 
> No functional change intended, except that, while at it,
> convert VM_BUG_* to VM_WARN_*.
> 
> Signed-off-by: Dev Jain <dev.jain@arm.com>
> ---

Yes, LGTM

Acked-by: David Hildenbrand (Arm) <david@kernel.org>

-- 
Cheers,

David

Re: [PATCH 2/5] mm/rmap: Add try_to_unmap_hugetlb_one

[David Hildenbrand (Arm)]

On 7/7/26 14:11, Dev Jain wrote:
> Simplify try_to_unmap_one() by separating the hugetlb parts into
> try_to_unmap_hugetlb_one().
> 
> To understand the correctness of the refactoring, the following points
> are noted:
> 
> 1. try_to_unmap() is called for hugetlb folios only when they are
>    hwpoisoned.
> 
> 2. A hugetlb VMA cannot be mlocked.
> 
> 3. page_vma_mapped_walk() returns at most one hugetlb mapping in a VMA,
>    and that mapping points at the head PFN.
> 
> 4. We won't ever process a softleaf entry that encodes a hugetlb folio;
>    hugetlb folios are never swapped out, migration entries will be
>    skipped (PVMW_MIGRATION not passed), and device-exclusive does not
>    work for hugetlb.
> 
> 5. The hwpoison entry is constructed from the poisoned folio, just as in
>    the pre-refactor code. Any previous uffd-wp state is deliberately not
>    preserved for the hwpoison entry.
> 
> 6. TTU_HWPOISON is always present; for it to not be present, either the
>    folio has to be in swapcache, or mapping_can_writeback() is true (see
>    unmap_poisoned_folio), none of which is true for hugetlb folios.
> 
> 7. Hugetlb uses separate counters from normal rss counters, therefore
>    update_highwater_rss() need not be called.
> 
> While at it:
> 
>  - Change VM_BUG_* to VM_WARN_*.
> 
>  - Do not declare variables which are only used once.
> 
>  - Use huge_pte_dirty() instead of pte_dirty().
> 
>  - Add 3 VM_WARN_ON_ONCE asserting that TTU_HWPOISON should be present,
>    pte_present() must be true, and the pfn derived from the huge pte
>    must be that of the head of the hugetlb folio.
> 
> Except the BUG->WARN change, no functional change intended.
> 
> Suggested-by: David Hildenbrand <david@kernel.org>
> Signed-off-by: Dev Jain <dev.jain@arm.com>
> ---
>  include/linux/hugetlb.h |   1 +
>  mm/rmap.c               | 177 +++++++++++++++++++++-------------------
>  2 files changed, 96 insertions(+), 82 deletions(-)
> 
> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> index 4115076e4922a..bf7e163e3779d 100644
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -1271,6 +1271,7 @@ static inline void hugetlb_count_sub(long l, struct mm_struct *mm)
>  }
>  
>  pte_t huge_ptep_get(struct mm_struct *mm, unsigned long addr, pte_t *ptep);
> +unsigned long huge_pte_dirty(pte_t pte);
>  
>  static inline pte_t huge_ptep_clear_flush(struct vm_area_struct *vma,
>  					  unsigned long addr, pte_t *ptep)
> diff --git a/mm/rmap.c b/mm/rmap.c
> index dbb077f8443e3..934773dfa2f2a 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1978,6 +1978,95 @@ static inline unsigned int folio_unmap_pte_batch(struct folio *folio,
>  				     FPB_RESPECT_WRITE | FPB_RESPECT_SOFT_DIRTY);
>  }
>  
> +static bool try_to_unmap_hugetlb_one(struct folio *folio,
> +		struct vm_area_struct *vma, unsigned long address, void *arg)
> +{
> +	DEFINE_FOLIO_VMA_WALK(pvmw, folio, vma, address, 0);
> +	struct mmu_notifier_range range;
> +	enum ttu_flags flags = (enum ttu_flags)(long)arg;
> +	struct mm_struct *mm = vma->vm_mm;
> +	const unsigned long hsz = huge_page_size(hstate_vma(vma));

I would move const variables all the way up.

> +	bool ret = true;
> +	pte_t pteval;
> +
> +	/*
> +	 * The try_to_unmap() is only passed a hugetlb folio in the case
> +	 * where the hugetlb folio is poisoned.
> +	 */
> +	VM_WARN_ON_FOLIO(!folio_test_hwpoison(folio), folio);
> +	VM_WARN_ON_ONCE(!(flags & TTU_HWPOISON));
> +
> +	range.end = vma_address_end(&pvmw);
> +	mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma->vm_mm,
> +				address, range.end);
> +	adjust_range_if_pmd_sharing_possible(vma, &range.start, &range.end);
> +	mmu_notifier_invalidate_range_start(&range);
> +
> +	/* There is only a single mapping in a VMA. */
> +	if (!page_vma_mapped_walk(&pvmw))
> +		goto range_end;
> +
> +	address = pvmw.address;

Will address ever change due to the page_vma_mapped_walk()? I don't think so, right?

So this can instead become a VM_WARN_ON_ONCE, maybe?



Nothing else jumped at me, much clearer to me.

Acked-by: David Hildenbrand (Arm) <david@kernel.org>

-- 
Cheers,

David

Re: [PATCH 3/5] mm/rmap: refactor some code around lazyfree folio unmapping

[David Hildenbrand (Arm)]

On 7/7/26 14:11, Dev Jain wrote:
> For lazyfree folio unmapping, after clearing the ptes we must abort the
> operation if the folio got dirtied or it has unexpected references.
> 
> Refactor this logic into a function which will return whether we need
> to abort or not.
> 
> If we abort, we restore the ptes and bail out of try_to_unmap_one.
> Otherwise adjust the rss stats of the mm and jump to a label.
> 
> Also rename that label from "discard" to "finish_unmap"; the former
> is appropriate in the lazyfree context, but the code following the label
> is executed for other successful unmap code paths too, so 'discard' does
> not sound correct for them.
> 
> No functional change intended.
> 
> Signed-off-by: Dev Jain <dev.jain@arm.com>
> ---
>  mm/rmap.c | 87 +++++++++++++++++++++++++++++++------------------------
>  1 file changed, 49 insertions(+), 38 deletions(-)
> 
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 934773dfa2f2a..00b571c2a1bab 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -2067,6 +2067,52 @@ static bool try_to_unmap_hugetlb_one(struct folio *folio,
>  	return ret;
>  }
>  
> +static inline bool ttu_anon_lazyfree_folio(struct vm_area_struct *vma,
> +		struct folio *folio)
> +{
> +	int ref_count, map_count;
> +
> +	/*
> +	 * Synchronize with gup_pte_range():
> +	 * - clear PTE; barrier; read refcount
> +	 * - inc refcount; barrier; read PTE
> +	 */
> +	smp_mb();
> +
> +	ref_count = folio_ref_count(folio);
> +	map_count = folio_mapcount(folio);
> +
> +	/*
> +	 * Order reads for page refcount and dirty flag
> +	 * (see comments in __remove_mapping()).
> +	 */
> +	smp_rmb();
> +
> +	if (folio_test_dirty(folio) && !(vma->vm_flags & VM_DROPPABLE)) {
> +		/*
> +		 * redirtied either using the page table or a previously
> +		 * obtained GUP reference.
> +		 */
> +		folio_set_swapbacked(folio);
> +		return false;
> +	}
> +
> +	if (ref_count != 1 + map_count) {
> +		/*
> +		 * Additional reference. Could be a GUP reference or any
> +		 * speculative reference. GUP users must mark the folio
> +		 * dirty if there was a modification. This folio cannot be
> +		 * reclaimed right now either way, so act just like nothing
> +		 * happened.
> +		 * We'll come back here later and detect if the folio was
> +		 * dirtied when the additional reference is gone.
> +		 */
> +		return false;
> +	}
> +
> +	return true;

You could simply do


/*
 * Additional references could be due to GUP or from a speculative
 * lookup. ...
 */
return ref_count == 1 + map_count;


Nothing else jumped at me

Acked-by: David Hildenbrand (Arm) <david@kernel.org>

-- 
Cheers,

David

Re: [PATCH 4/5] mm/rmap: refactor anon swapbacked folio unmap in try_to_unmap_one

[David Hildenbrand (Arm)]

On 7/7/26 14:11, Dev Jain wrote:
> Refactor anonymous swapbacked folio unmap to ttu_anon_swapbacked_folio().
> 
> No functional change intended.
> 
> Signed-off-by: Dev Jain <dev.jain@arm.com>
> ---
>  mm/rmap.c | 105 ++++++++++++++++++++++++++++++++----------------------
>  1 file changed, 62 insertions(+), 43 deletions(-)
> 
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 00b571c2a1bab..ade78df5be2bd 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -2113,6 +2113,64 @@ static inline bool ttu_anon_lazyfree_folio(struct vm_area_struct *vma,
>  	return true;
>  }
>  

BTW, no need for the "inline" for most of these functions (applies to other
patches as well).

> +static inline void set_swp_pte_at(struct mm_struct *mm, unsigned long address,
> +		pte_t *ptep, swp_entry_t entry, pte_t pteval, bool anon_exclusive)

It's confusing that we say "set_swp_pte_at", but the pte_t we was is actually
the old pte.

Can we instead have a function that creates us a swap_pte from the other data,
and then do the set_pte_at() in the caller?

static pte_t swp_pte_prepare(...)
{
	...
	return swp_pte;
}

> +{
> +	pte_t swp_pte = swp_entry_to_pte(entry);
> +
> +	if (anon_exclusive)
> +		swp_pte = pte_swp_mkexclusive(swp_pte);
> +
> +	if (likely(pte_present(pteval))) {
> +		if (pte_soft_dirty(pteval))
> +			swp_pte = pte_swp_mksoft_dirty(swp_pte);
> +		if (pte_uffd_wp(pteval))
> +			swp_pte = pte_swp_mkuffd_wp(swp_pte);
> +	} else {
> +		/* Device-exclusive entry */
> +		if (pte_swp_soft_dirty(pteval))
> +			swp_pte = pte_swp_mksoft_dirty(swp_pte);
> +		if (pte_swp_uffd_wp(pteval))
> +			swp_pte = pte_swp_mkuffd_wp(swp_pte);
> +	}
> +
> +	set_pte_at(mm, address, ptep, swp_pte);
> +}
> +
> +static inline bool ttu_anon_swapbacked_folio(struct vm_area_struct *vma, struct folio *folio,
> +		struct page *subpage, unsigned long address, pte_t *ptep,

Subpages do not exist :)

> +		pte_t pteval)
> +{
> +	bool anon_exclusive = folio_test_anon(folio) && PageAnonExclusive(subpage);

const?

> +	swp_entry_t entry = page_swap_entry(subpage);
> +	struct mm_struct *mm = vma->vm_mm;
> +
> +	if (folio_dup_swap(folio, subpage) < 0)
> +		return false;
> +
> +	/*
> +	 * arch_unmap_one() is expected to be a NOP on
> +	 * architectures where we could have PFN swap PTEs,
> +	 * so we'll not check/care.
> +	 */
> +	if (arch_unmap_one(mm, vma, address, pteval) < 0) {
> +		folio_put_swap(folio, subpage);
> +		return false;
> +	}
> +
> +	/* See folio_try_share_anon_rmap(): clear PTE first. */
> +	if (anon_exclusive && folio_try_share_anon_rmap_pte(folio, subpage)) {
> +		folio_put_swap(folio, subpage);

I recall I stumbled over this before, but there is no way to undo the
arch_unmap_one(), right?

> +		return false;
> +	}
> +
> +	mm_prepare_for_swap_entries(mm);
> +	dec_mm_counter(mm, MM_ANONPAGES);
> +	inc_mm_counter(mm, MM_SWAPENTS);
> +	set_swp_pte_at(mm, address, ptep, entry, pteval, anon_exclusive);
> +	return true;
> +}
> +

-- 
Cheers,

David

Re: [PATCH 5/5] mm/rmap: add anon folio unmap dispatcher function

[David Hildenbrand (Arm)]

On 7/7/26 14:11, Dev Jain wrote:
> Add ttu_anon_folio() as the common entry point for anonymous folio
> unmapping. It dispatches to the lazyfree or swapbacked helper as
> appropriate, and centralizes restoration of cleared PTEs on failure.
> 
> No functional change intended.
> 
> Signed-off-by: Dev Jain <dev.jain@arm.com>
> ---
>  mm/rmap.c | 59 +++++++++++++++++++++++++++++++++----------------------
>  1 file changed, 36 insertions(+), 23 deletions(-)
> 
> diff --git a/mm/rmap.c b/mm/rmap.c
> index ade78df5be2bd..f021ecd51a4a2 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -2171,6 +2171,40 @@ static inline bool ttu_anon_swapbacked_folio(struct vm_area_struct *vma, struct
>  	return true;
>  }
>  
> +static inline bool ttu_anon_folio(struct vm_area_struct *vma, struct folio *folio,
> +		struct page *subpage, unsigned long address, pte_t *ptep,
> +		pte_t pteval, unsigned long nr_pages)
> +{
> +	struct mm_struct *mm = vma->vm_mm;
> +	bool ret;
> +
> +	/*
> +	 * Store the swap location in the pte.
> +	 * See handle_pte_fault() ...
> +	 */
> +	if (unlikely(folio_test_swapbacked(folio) !=
> +			folio_test_swapcache(folio))) {
> +		WARN_ON_ONCE(1);
> +		return false;
> +	}
> +
> +	/* MADV_FREE page check */
> +	if (!folio_test_swapbacked(folio)) {
> +		ret = ttu_anon_lazyfree_folio(vma, folio);
> +		if (ret)
> +			add_mm_counter(mm, MM_ANONPAGES, -nr_pages);

Can't the be handled in there as well?

> +	} else {
> +		/* nr_pages > 1 not supported yet */
> +		ret = ttu_anon_swapbacked_folio(vma, folio, subpage, address,
> +						ptep, pteval);
> +	}
> +
> +	if (!ret)
> +		set_ptes(mm, address, ptep, pteval, nr_pages);

It might be cleaner to do that in the caller, where we actually removed the ptes
in the first place?

> +
> +	return ret;
> +}
> +
>  /*
>   * @arg: enum ttu_flags will be passed to this argument
>   */
> @@ -2351,31 +2385,10 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
>  			 */
>  			dec_mm_counter(mm, mm_counter(folio));
>  		} else if (folio_test_anon(folio)) {
> -			/*
> -			 * Store the swap location in the pte.
> -			 * See handle_pte_fault() ...
> -			 */
> -			if (unlikely(folio_test_swapbacked(folio) !=
> -					folio_test_swapcache(folio))) {
> -				WARN_ON_ONCE(1);
> +			if (!ttu_anon_folio(vma, folio, subpage, address,
> +					    pvmw.pte, pteval, nr_pages))
>  				goto walk_abort;
> -			}
>  
> -			/* MADV_FREE page check */
> -			if (!folio_test_swapbacked(folio)) {
> -				if (!ttu_anon_lazyfree_folio(vma, folio)) {
> -					set_ptes(mm, address, pvmw.pte, pteval, nr_pages);
> -					goto walk_abort;
> -				}
> -				add_mm_counter(mm, MM_ANONPAGES, -nr_pages);
> -				goto finish_unmap;
> -			}
> -
> -			if (!ttu_anon_swapbacked_folio(vma, folio, subpage, address,
> -						pvmw.pte, pteval)) {
> -				set_pte_at(mm, address, pvmw.pte, pteval);
> -				goto walk_abort;
> -			}
>  			goto finish_unmap;
>  		} else {
>  			/*

Very nice.

-- 
Cheers,

David